EVOLUTION AND MORPHOGENETIC RULES: THE SHAPE OF THE VERTEBRATE LIMB IN ONTOGENY AND PHYLOGENY

The notion of a "developmental constraint" has become a catchphrase for a collection of poorly defined notions about how ontogeny affects phylogeny. In this paper, we shall attempt to define this idea more precisely by examining the vertebrate limb from three viewpoints. First, theoretical models of morphogenesis suggest several generalizations about how limb geometry is laid down during development. Comparative studies and experimental manipulations of developing limbs independently confirm these generalizations, which amount to a set of "construction rules" for determining how the major features of limb architecture are established in ontogeny. Armed with these rules, we can inquire how limb morphology can be varied during evolution and suggest a more precise operational definition of "developmental constraints" on morphological evolution.